The present invention relates to a damping force controller for controlling a damping force characteristic of a damper disposed between an unsprung mass and a sprung mass of a vehicle, and more particularly to a damping force controller with a fail-safe function.
Japanese Patent Application First (unexamined) Publication No. 64-60411 discloses a system for controlling a damping force of a shock absorber disposed between an unsprung mass and a sprung mass of a vehicle. According to this known system, a load sensor of the piezoelectric type is disposed between a shock absorber and a suspension upper support of a mount insulator. An output of the load sensor is supplied to a control unit where a damping force vs., piston velocity characteristic of the shock absorber is stored. The output of the load sensor is used to retrieve the stored characteristic to give a velocity of the piston of the shock absorber. The velocity obtained is integrated to give a relative displacement. The relative displacement thus calculated is used in determining which of a plurality of damping force ranges is to be selected. In accordance with this result, the shock absorber is shifted to the damping force range selected. This system is not satisfactory in that should if the load sensor or its harness fail to operate normally, the shock absorber continues to produce a low damping force, resulting in degraded running stability of the vehicle.
An object of the present invention is to improve a damping force controller such that even if a load sensor or its harness fail to operate normally, running stability of the vehicle is maintained.